The invention is more particularly directed to a printing apparatus of said type, which is named a "toner-jet" printing apparatus, and in which a dry colour powder, generally named "toner", is, by a direct method, transferred from a rotating toner feeder roll, through apertures of a fixed matrix in the form of a flexible printing circuit and to the object to be printed, for instance the paper, which is moved over a support roll, and in which the toner received on the paper is finally fixed on the paper by a heat treatment.
The principle of said process is that there are created two electric fields for transferring the toner from the feeder roll to the paper, a first electric field between the toner feeder roll and the toner matrix, which field can be brought to invert its polarity, and a second electric field, preferably a constantly downwards directed positive electric field between the matrix and the support roll over which the paper is conveyed.
The toner matrix is formed with a large number of very narrow, through apertures having a diameter of for instance 100-300 .mu.m, and around each such aperture an electrically conducting ring of a suitable metal, for instance copper, in the following referred to as "copper ring". Each copper ring is arranged so that a positive potential, for instance +300 V, can be impressed thereto, which potential is higher than the potential of the feeder roll, which can be for instance between +5 and +100 V, preferably about +50 V, but which is lower than the potential of the support roll for the paper, which can be for instance +1500 V. The electrically conducting ring, when impressed with a voltage, makes the belonging matrix aperture become "opened" for letting through toner. If, on the contrary, the matrix aperture is given a potential which is substantially less than the potential of the toner feeder roll, for instance if it is connected to earth the belonging matrix aperture becomes "closed" thereby preventing toner from passing down through said aperture.
The function is as follows:
the colour powder (toner) gets a negative potential in that the toner particles are rubbed against each other; PA1 the toner is supplied to the toner feeder roll, which has a positive charging of a predetermined potential, often a potential which can be varied between +0 and +100 V, and the toner is spread in an even, suitably thick layer on the feeder roll by means of a doctor blade; PA1 each aperture of the matrix which corresponds to a desired toner point is opened in that the matrix aperture ring is impressed by a positive potential which is higher that the potential of the feeder roll, for instance +300 V; apertures corresponding to non-toner-carrying portions remain connected to earth, which means that said apertures are to be considered as "closed" and that they thereby make it impossible for toner to pass said apertures; the combination of opened matrix apertures create a sign to be imaged; PA1 depending on the difference in potential, for instance +50 V to +300 V=+250 V between the feeder roll and the toner matrix the negatively charged toner particles are sucked down from the feeder roll to the matrix, and depending on the difference in potential between the toner matrix and the support roll mounted underneath same, for instance +300 V to +1500 V=+1200 V toner particles are moved from the matrix and deposit on the paper conveyed over the support roll; PA1 the paper having toner deposited thereon is finally moved through a heat treatment apparatus in which the toner is fixed to the paper.
There is an almost linear relationship between the current density and the traction force that the electric field exerts on the toner particles. The greatest density of the field is located very close above the copper rings and the density decreases in the direction towards the centre of the aperture. By reducing the potential of the feeder roll and thereby increasing the difference in potential between the feeder roll and the matrix it is possible to increase the amount of toner which is allowed to pass same; an increase of the potential of the feeder roll provides a corresponding reduction of the amount of toner which is let through.
By connecting a copper ring of the matrix to earth the direction of potential is inverted between the feeder roll from having been +250 V in the direction downwards to be +50 V in the direction upwards, and this makes negatively charged toner particles stick to the feeder roll, or makes such particles become sucked back thereto, respectively.
In a particular embodiment of a printing apparatus the distance between the feeder roll and the matrix was adjusted to about 0.1 mm, and the distance between the matrix and the support roll to about 0.6 mm. For the above mentioned potentials, which are given as examples, this gives a field strength of 2.5 V/.mu.m, which is higher that the insulation property of air, which is about 1 V/.mu.m. For eliminating the risque of flash-over between the feeder roll and the copper ring of the matrix and between the copper ring and the support roll it is therefore necessary that the matrix aperture ring be insulated.
In printing apparatus of toner-jet type, so far known, the copper rings have been insulated by being "baked into" (embedded in) the matrix material, and therefore the inner diameter of the copper ring of the matrix aperture has been made greater than the diameter of the matrix aperture, and an insulation material has been applied so as to cover all sides of the matrix. For a matrix aperture having a diameter of for instance 190 .mu.m the inner diameter of the copper ring was made 250 .mu.m. This means that the matrix aperture for letting toner down has a surface which is only 57.8% of the surface inside the copper ring, and the aperture for letting toner through is located some distance radially inside the inner diameter of the copper ring, where the field density is highest and should have given maximum force for sucking toner down. As a consequence there is a highly restricted degree of toner supply.